1. Field of the Invention
The present invention relates to both a method and an apparatus for controlling the propagation of magnetic fields by electrodynamic/magnetic transducers, in telecommunication devices wherein static magnetic fields produced by the transducers are substantially shielded and dynamic magnetic fields are radiated in a substantially unimpeded manner.
2. Description of the Prior Art
Electrodynamic/magnetic transducers which are a subgroup of electroacoustic transducers, are used wherever electrical or electronic signals are to be converted into speech and/or speech is to be converted into electrical or electronic signals. Typical fields of use are, therefore, the audio and hi-fi field, domestic engineering in areas where alarm and bell signals are output, and telecommunications engineering, for example.
In the latter field of use, the electrodynamic transducers are particularly used, for example, in handsets (cord-connected, cordlessxe2x80x94e.g., mobile parts and mobile phones), headphones and headsets, usually in the form of earphones, but also sometimes in the form of a microphone. The use of electromagnetic transducers is less common.
A big disadvantage of these transducer types, particularly the electrodynamic transducers, is that they produce, as shown in FIG. 1 (showing a basic sketch of an electrodynamic transducer a), static magnetic fields (stray fields) MFs as a result of a pot magnet TM, for example, wherein the magnetic fields penetrate non-magnetic materials (plastics) unimpeded. Magnetizable objects such as pins, paperclips, iron filings, particles (in industries which use iron or steel, metalworking shops, etc., are inevitably attracted toward the center of the transducer. If the particles are small enough to pass through the (voice inlet) voice outlet openings, they collect at the place of greatest field strength (clearance of the pot magnet TM) and permanently jam a diaphragm MB of the transducer. Depending on how sensitive the diaphragm MB, is and hence the transducer as such, toward such mini foreign bodies, the result is either an abrupt total failure or a gradual failure of the diaphragm MB.
Furthermore, the relative movement of the handset in the vicinity of electrical conductors, particularly inductors, results in unwanted induced currents (a key issue regarding cardiac pacemakers, medical devices etc.).
As a result of the devices being miniaturized (e.g., characterized by a short distance between the transducer""s sound exit opening and the sound exit opening in the housing of the handset; cf. FIG. 1), smaller and smaller cordless mobile parts or mobile radio phones are becoming available on the market. However, problems are intensifying because xe2x80x9crare earthxe2x80x9d magnets (such as magnets made of Nd or Sm alloys) with relatively high remanences, and, hence, relatively strong stray fields, are frequently being used in flat and small transducers.
The result of the problems outlined is, on the one hand, in some countries (e.g., Australia, Great Britain, USA) licensing requirements limit the static magnetic field. On the other hand, there recently has been an increased number of cases, particularly with GSM mobile phones, arising from earphones having failed on account of jammed diaphragms. Although fine-meshed tissue (e.g., dust webs) in the acoustic openings prevent the diaphragm from becoming jammed, they clogged over time such that reproduction became continuously quieter as the magnetic force is permanently exerted on the magnetizable particles contained in the tissue.
To solve this problem, electrets and piezoelectric microphones, which represent equivalent alternatives, are called upon for implementing the microphones in the handsets.
The situation is different, however, for implementing the earphones.
Piezoelectric transducers used as earphones have no pronounced magnetic field. However, the transducer technology based on the piezoelectric effect has two clear disadvantages as compared with the transducer technology based on a magnetic field: 1) From the point of view of speech quality, electrodynamic transducers, particularly with small diameters, are clearly superior; 2) Some countries (e.g., Australia, Great Britain, USA, Italy) and, in addition, British Telecom and France Telecom, generally have the requirement of xe2x80x9chearing aid compatibilityxe2x80x9d (hac) for stimulating hearing aids. This stimulation is (virtually) exclusively inductive and is based on a dynamic magnetic field (alternating field). This means that the required (measurement of the alternating field in an hac measurement plane as shown in FIG. 1) magnetic alternating field needs to be produced using cumulative supplementary coils.
Thus, in the end, the electrodynamic ransducers, in particular, are again called upon after all and, at the same time, attempts are made to solve the aforementioned problem in a different way.
When electrodynamic transducers are used, it is naturally helpful, for reducing the static stray fields, to have a relatively large distance between the transducer""s sound exit opening and the surface of the handset as has been done, for example, in the, handset of the cord-connected Siemens xe2x80x9cSymphony Dxe2x80x9d devices available on the market. However, this procedure is absolutely contrary to the market requirements of smaller and smaller handsets, particularly mobile (cordless) handsets. To solve this problem, the xe2x80x9ctransducer""s sound exit opening ⇄ surface of the handsetxe2x80x9d distance would have to be approximately 1 cm. This would allow the aforementioned negative influences of the static magnetic field to be eliminated.
However, by increasing the distance, a dynamic magnetic field MFd (as shown in FIG. 2xe2x80x94a basic sketch of an electrodynamic transducer), which is needed on account of the hac requirement and is also large enough given proficient dimensioning of a plunger-type coil TS which produces this dynamic magnetic field MFd, in conventional electrodynamic transducers, is also attenuated to such an extent that it is no longer sufficient for the hac requirement. This means that additional air-core coils are again necessary here, too, to amplify the alternating field.
Taking FIG. 1 as a basis, a cover AD (shield, e.g.,) in the form of a shielding plate) is therefore used, as shown in FIG. 3, in a familiar manner. This cover concentrates the field lines of the stray field MFs and allows them less projection into space. The cover must, of course, be provided with openings to be xe2x80x9ctransparentxe2x80x9d to the sound pressure produced by the diaphragm MB. The magnetic alternating fields MFd (cf. FIG. 2) for the hac requirement are consequently attenuated again, however, and therefore need to be produced with supplementary coils fitted in front of the cover.
EP 0 422 424 A2 discloses an electrodynamic transducer with improved electrical and magnetic shielding, which has a moving coil, a diaphragm and a magnet system which are surrounded by a housing with a cover. Clamped between the inside of the cover and the internal pole plate of the magnet system, there is an insert which is made of a nonmagnetizable, electrically insulating material. The insert defines the axial position of the magnet system with respect to the cover, and the cover shields external magnetic alternating fields and has openings which let through a sound pressure.
DE 3 401 072 A1 discloses an electrodynamic transducer head for nondestructive testing of workpieces using ultrasound, wherein the transducer head has an electromagnet which has a magnetic yoke and includes an outer pole shoe and an inner pole shoe surrounded by the latter. These pole shoes have exciter and reception coils arranged on them. To allow the inner pole shoe to be placed directly on the workpiece, moving relative to the latter, without any risk, these exciter and reception coils are provided with a protective cap which is surrounded by a protective ring whose end face facing the workpiece projects over that of the protective cap, the inner pole shoe and/or the protective cap having radially extending slits for preventing eddy currents, and the slits letting through a sound pressure.
An object to which the present invention is directed is that of improving the propagation of magnetic fields by electrodynamic/magnetic transducers, particularly in telecommunication devices, such that static magnetic fields produced by the transducers are substantially shielded and dynamic magnetic fields are radiated in a substantially unimpeded manner.
The idea on which the present invention is based is that of providing a magnetic cover (shield), e.g. for technical production reasons, in the form of a shielding plate or deep-drawn plate which is substantially impermeable to static magnetic fields from an electrodynamic/magnetic transducer, at least in the sound exit region of the transducer. The cover can be inserted, for example while the transducer is being mounted between the housing which surrounds the transducer (e.g., the handset shell) and the transducer, or it can be premounted on the transducer in the form of a pot.
The cover has a number of openings passing through the cover, the number of openings being designed to be such that;
(1) the cover provided with the openings still has a predetermined strength or stiffness with respect to its original strength or stiffness and is, thus, still mechanically mountable;
(2) the openings let through a sound pressure level which can be produced by the transducer; and
(3) the openings spatially delimit eddy currents on the cover which are produced by dynamic magnetic fields from the transducer, which means that these eddy currents cannot develop to full strength.
In summary, the cover includes a particular combination of geometric and magnetic material properties.
In an embodiment of the present invention the spatial delimitation can be achieved in that the openings cross the eddy currents which would be induced without them in the cover.
In terms of type and number, the openings fulfill the listed criteria (1) . . . (3) quite well, in an alternative embodiment particularly when they are arranged or run substantially radially on the cover.
In another embodiment, it is advantageous if the cover is slit; that is if the openings are in the form of slits, for example.
For the arrangement of the openings on the cover, it is advantageous if the opening(s) are arranged in the center of the covers on account of the concentration properties of eddy currents (concentration of eddy currents), particularly with rotationally symmetrical covers.
In accordance with a further embodiment, at least one of the slits is substantially the same length as the length of the cover at its maximum extent.
pursuant to yet another embodiment it is advantageous if the magnetic cover is soft magnetic, or has a low coercive force.
Introducing a magnetic material into a magnetic field, as in the present case, for example, produces a gain in the dynamic magnetic field component present in the magnetic field if the substance of the material (in the present case the substance of the cover) with regard to the magnetic field containing the material is coordinated with one another such that, under these circumstances, the point under consideration is the steepest rise in the new curve for the hysteresis curve. In the present case (use of, in particular, electrodynamic transducers in telecommunication terminals, where the dynamic magnetic field produced by the plunger-type coil in the dynamic transducer is utilized for stimulating hearing aids (the key issue being hearing aid compatibility), this gain is therefore desirable because it means that other measures for amplifying the dynamic magnetic field, e.g. using additional air-core coils, are not necessary.
In another embodiment of the present invention it is therefore advantageous if the magnetic cover is made of a material such that, with the predetermined magnetic field from the transducer, a maximum gain is produced for the dynamic magnetic field let through by the cover.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Preferred Embodiments and the Drawings.